There are known arrangements for tuning a laser to operate at selected frequencies throughout a wide range of frequencies. Frequency-tunable semiconductor lasers are attractive devices for optical frequency division multiplex transmission systems. In a tunable arrangement, a distributed Bragg reflector laser provides a very large tuning range when different values of injection, or bias, current are injected into the phase control and the distributed Bragg reflector regions of the laser.
In the prior art, changes of frequency have been made by selecting the values of bias current from continuously variable analog functions. Although the tuning range is very wide, there is a problem when the laser is changed from a first frequency to a second frequency and thereafter is to be returned to the exact first frequency. In returning the laser to the first frequency, it is very difficult to select the injection current value which will produce the exact first frequency. The exact frequencies are defined by the resonances of a Fabry-Perot resonator. There is no provision in the prior art to measure the drift of the operating characteristic of the laser.